Vitamin K-dependent proteins contain 9 to 13 gamma-carboxyglutamic acid residues (Gla) in their amino terminal 45 residues. The Gla residues are produced by enzymes in the liver that utilize vitamin K to carboxylate the side chains of glutamic acid residues in protein precursors. Vitamin K-dependent proteins are involved in a number of biological processes, of which the most well described is blood coagulation (reviewed in Furie and Furie, 1988, Cell, 53:505–518). Vitamin K-dependent proteins include protein Z, protein S, prothrombin, factor X, factor IX, protein C, factor VII, and Gas6. The latter protein functions in cell growth regulation. Matsubara et al., 1996, Dev. Biol., 180:499–510. The Gla residues are needed for proper calcium binding and membrane interaction by these proteins. The membrane contact site of factor X is thought to reside within amino acid residues 1–37. Evans and Nelsestuen, 1996, Protein Sci., 5:suppl. 1, 163 Abs. Although the Gla-containing regions of the plasma proteins show a high degree of sequence homology, they have at least a 1000-fold range in membrane affinity. McDonald et al., 1997, Biochemistry, 36:5120–5137.
Factor VII functions in the initial stage of blood clotting and may be a key element in forming blood clots. The inactive precursor, or zymogen, has low enzyme activity that is greatly increased by proteolytic cleavage at the R152I153 bond to form factor VIIa. This activation can be catalyzed by factor Xa as well as by Vila-tissue factor, an integral membrane protein found in a number of cell types. Fiore et al., 1994, J. Biol. Chem., 269:143–149. Activation by VIIa-tissue factor is referred to as autoactivation. It is implicated in both the activation (formation of factor VIIa from factor VII) and the subsequent activity of factor VIIa. The most important pathway for activation in vivo is not known. Factor VIIa can activate blood-clotting factors IX and X.
Tissue factor is expressed at high levels on the surface of some tumor cells. A role for tissue factor, and for factor VIIa, in tumor development and invasion of tissues is possible. Vrana et al., Cancer Res., 56:5063–5070. Cell expression and action of tissue factor is also a major factor in toxic response to endotoxic shock. Dackiw et al., 1996, Arch. Surg., 131:1273–1278.
Protein C is activated by thrombin in the presence of thrombomodulin, an integral membrane protein of endothelial cells. Esmon et al., 1982, J. Biol. Chem., 257:859–864. Activated protein C (APC) degrades factors Va and VIIIa in combination with its cofactor, protein S. Resistance to APC is the most common form of inherited thrombosis disease. Dahlback, 1995, Blood, 85:607–614. Vitamin K inhibitors are commonly administered as a prophylaxis for thrombosis disease.
Vitamin K-dependent proteins are used to treat certain types of hemophilia. Hemophilia A is characterized by the absence of active factor VIII, factor VIIIa, or the presence of inhibitors to factor VIII. Hemophilia B is characterized by the absence of active factor IX, factor IXa. Factor VII deficiency, although rare, responds well to factor VII administration. Bauer, 1996, Haemostasis, 26:155–158, suppl. 1. Factor VIII replacement therapy is limited due to development of high-titer inhibitory factor VIII antibodies in some patients. Alternatively, factor VIIa can be used in the treatment of hemophilia A and B. Factor IXa and factor VIIIa activate factor X. Factor VIIa eliminates the need for factors IX and VIII by activating factor X directly, and can overcome the problems of factor IX and VIII deficiencies with few immunological consequences. Hedner et al., 1993, Transfus. Medi. Rev., 7:78–83; Nicolaisen et al., 1996, Thromb. Haemost., 76:200–204. Effective levels of factor VIIa administration are often high (45 to 90 μg/kg of body weight) and administration may need to be repeated every few hours. Shulmav et al., 1996, Thromb. Haemost., 75:432–436.
A soluble form of tissue factor (soluble tissue factor or sTF) that does not contain the membrane contact region has been found to be efficacious in treatment of hemophilia when co-administered with factor VIIa. See, for example, U.S. Pat. No. 5,504,064. In dogs, sTF was shown to reduce the amount of factor VIIa needed to treat hemophilia. Membrane association by sTF-VIIa is entirely dependent on the membrane contact site of factor VII. This contrasts to normal tissue-factor VIIa complex, which is bound to the membrane through both tissue factor and VII (a).